In a conventional cable television converter the incoming television signal, which includes both video- and audio-modulated carriers for each channel, is subjected to a dual-conversion process, by means of which the incoming signal is first converted to an intermediate-frequency signal, and then in a second step this intermediate-frequency signal is converted to an output frequency corresponding to that of a locally unused television channel, typically channel 3. In accordance with FCC regulations, the video and audio carriers for each channel must be accurately separated by 4.5 MHz at a narrow tolerance of .+-.1 KHz.
In various applications, such as in pay television cable systems, the incoming video and audio signals are demodulated in the converter to baseband frequencies to permit the video and audio signals to be descrambled and, in some instances, to allow the volume of the audio signal to be remotely controlled. After the audio and video signals have been descrambled, they modulate audio and video carriers which are internally developed at the converter. The thus modulated reconstituted audio and video carriers are then mixed or combined and applied to the subscriber's television receiver at the preselected unused channel.
In order to meet the aforementioned FCC spacing requirements between the audio and video carriers, the restored or remodulated video and audio carriers must be accurately spaced by 4.5 MHz.+-.1 KHz. In the conventional arrangement the modulated audio carrier, which is an FM signal, is generated by a free-running, variable-frequency LC oscillator and the video carrier is typically generated by a crystal oscillator. The known circuits used to generate the video and audio carriers are, however, not able to reliably maintain the .+-.1 KHz tolerance in the required 4.5 MHz spacing between these carriers, largely because these circuits are not stable with respect to temperature and time. There thus exists a need for a circuit which is capable of producing two radio-frequency signals accurately and reliably spaced in frequency from one another with a narrow tolerance, in the order of .+-.1 KHz, such as for use as audio and video carriers in a cable pay-television converter, and which can be fabricated at a moderate cost.
It is therefore an object of the invention to provide a circuit for producing two carrier signals which are accurately separated by a predetermined frequency.
It is a further object of the invention to provide a circuit of the type described in which accurate spacing of two carrier signals can be achieved in a circuit employing a single low-cost crystal.
It is an additional object of the present invention to provide a low-cost circuit for use in a pay-television converter in which the FCC-required precise tolerance in the spacing between the audio and video carriers is reliably attained.